Broadcast and multicast routing

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sourceduplication

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in-networkduplication

duplicatecreation/transmissionduplicate

duplicate

Broadcast Routing Deliver packets from srce to all other nodes Source duplication is inefficient:

Source duplication: how does source determine recipient addresses

In-network duplication Flooding: when node receives brdcst

pckt, sends copy to all neighbors Problems: cycles & broadcast storm

Controlled flooding: node only brdcsts pkt if it hasn’t brdcst same packet before Node keeps track of pckt ids already

brdcsted Or reverse path forwarding (RPF): only

forward pckt if it arrived on shortest path between node and source

Spanning tree No redundant packets received by any

node

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B

G

D

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F

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B

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D

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(a) Broadcast initiated at A (b) Broadcast initiated at D

Spanning Tree First construct a spanning tree Nodes forward copies only along

spanning tree

Spanning Tree: Creation Center node Each node sends unicast join message to

center node Message forwarded until it arrives at a node

already belonging to spanning tree

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(a) Stepwise construction of spanning tree

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(b) Constructed spanning tree

Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting

routers having local mcast group members tree: not all paths between routers used source-based: different tree from each sender

to rcvrs shared-tree: same tree used by all group

members

Shared tree Source-based trees

Approaches for building mcast treesApproaches: source-based tree: one tree per source

shortest path trees reverse path forwarding

group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees

…we first look at basic approaches, then specific protocols adopting these approaches

Shortest Path Tree

mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra’s algorithm

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i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source

Reverse Path Forwarding

if (mcast datagram received on incoming link on shortest path back to center)

then flood datagram onto all outgoing links

else ignore datagram

rely on router’s knowledge of unicast shortest path from it to sender

each router has simple forwarding behavior:

Reverse Path Forwarding: example

• result is a source-specific reverse SPT– may be a bad choice with asymmetric

links

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datagram will be forwarded

LEGENDS: source

datagram will not be forwarded

Reverse Path Forwarding: pruning forwarding tree contains subtrees with no

mcast group members no need to forward datagrams down

subtree “prune” msgs sent upstream by router

with no downstream group members

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prune message

LEGENDS: source

links with multicastforwarding

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P

P

Shared-Tree: Steiner Tree

Steiner Tree: minimum cost tree connecting all routers with attached group members

problem is NP-complete excellent heuristics exists not used in practice:

computational complexity information about entire network

needed monolithic: rerun whenever a router

needs to join/leave

Center-based trees

single delivery tree shared by all one router identified as “center” of

tree to join:

edge router sends unicast join-msg addressed to center router

join-msg “processed” by intermediate routers and forwarded towards center

join-msg either hits existing tree branch for this center, or arrives at center

path taken by join-msg becomes new branch of tree for this router

Center-based trees: an example

Suppose R6 chosen as center:

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path order in which join messages generated

LEGEND

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Internet Multicasting Routing: DVMRP DVMRP: distance vector multicast

routing protocol, RFC1075 flood and prune: reverse path

forwarding, source-based tree RPF tree based on DVMRP’s own routing

tables constructed by communicating DVMRP routers

no assumptions about underlying unicast initial datagram to mcast group flooded

everywhere via RPF routers not wanting group: send upstream

prune msgs

DVMRP: continued… soft state: DVMRP router periodically (1

min.) “forgets” branches are pruned: mcast data again flows down unpruned branch downstream router: reprune or else continue

to receive data routers can quickly regraft to tree

following IGMP join at leaf odds and ends

commonly implemented in commercial routers Mbone routing done using DVMRP

Tunneling

Q: How to connect “islands” of multicast routers in a “sea” of unicast routers?

mcast datagram encapsulated inside “normal” (non-multicast-addressed) datagram

normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router

receiving mcast router unencapsulates to get mcast datagram

physical topology logical topology

PIM: Protocol Independent Multicast

not dependent on any specific underlying unicast routing algorithm (works with all)

two different multicast distribution scenarios :

Dense: group members

densely packed, in “close” proximity.

bandwidth more plentiful

Sparse: # networks with group

members small wrt # interconnected networks

group members “widely dispersed”

bandwidth not plentiful

Consequences of Sparse-Dense Dichotomy:

Dense group membership

by routers assumed until routers explicitly prune

data-driven construction on mcast tree (e.g., RPF)

bandwidth and non-group-router processing profligate

Sparse: no membership until

routers explicitly join

receiver- driven construction of mcast tree (e.g., center-based)

bandwidth and non-group-router processing conservative

PIM- Dense Mode

flood-and-prune RPF, similar to DVMRP but

underlying unicast protocol provides RPF info for incoming datagram

less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm

has protocol mechanism for router to detect it is a leaf-node router

PIM - Sparse Mode center-based

approach router sends join

msg to rendezvous point (RP) intermediate routers

update state and forward join

after joining via RP, router can switch to source-specific tree increased

performance: less concentration, shorter paths

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join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

PIM - Sparse Mode

sender(s): unicast data to RP,

which distributes down RP-rooted tree

RP can extend mcast tree upstream to source

RP can send stop msg if no attached receivers “no one is

listening!”

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join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

PIM-SM(1)

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Receiver 2

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A B RP D

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PIM-SM(2)

Receiver 1

Source

Receiver 2

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Receiver 1 Joins Group GC Creates (*, G) State, Sends(*, G) Join to the RP

Join

PIM-SM(3)

Receiver 1

Source

Receiver 2

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A B RP D

C E

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RP Creates (*, G) State

PIM-SM(4)

Receiver 1

Source

Receiver 2

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A B RP D

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Source Sends DataA Sends Registration to the RP

Register

Data

IP tunnel between A and RP sincemulticast tree is not established

PIM-SM(5)

Receiver 1

Source

Receiver 2

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A B RP D

C E

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RP decapsulates RegistrationForwards Data Down the Shared TreeSends Joins Towards the Source

joinjoin

PIM-SM(6)

Receiver 1

Source

Receiver 2

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A B RP D

C E

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RP Sends Register-Stop OnceData Arrives Natively

Register-Stop

PIM-SM(7)SPT Switchover

Receiver 1

Source

Receiver 2

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A B RP D

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C Sends (S, G) Joins to Join theShortest Path Tree (SPT)

join

PIM-SM(8)

Receiver 1

Source

Receiver 2

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A B RP D

C E

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C starts receiving Data natively

PIM-SM(9)

Receiver 1

Source

Receiver 2

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A B RP D

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C Sends Prunes Up the RP tree forthe Source. RP Deletes (S, G) OIF andSends Prune Towards the Source

Prune

PrunePrune

PIM-SM(10)

Receiver 1

Source

Receiver 2

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C E

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B, RP pruned

PIM-SM(11)

Receiver 1

Source

Receiver 2

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A B RP D

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join

New receiver2 joinsE Creates State and Sends (*, G) Join

PIM-SM(12)

Receiver 1

Source

Receiver 2

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A B RP D

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C Adds Link Towards E to the OIFList of Both (*, G) and (S, G)Data from Source Arrives at E

Broadcast and multicast routing

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